Exchange 2010 DAG Site to Site Configuration

Hi I have a client who currently has 2 Windows Server 2008 Ent machines and we have setup a site to site VPN connection which works perfect, however they are currently only running exchange 2010 Std on the server in there main office the other server is just currently being used for DFS-R.
Basically what they want is a failover site as they are out on the road a lot so incase of the internet going down at the main site the passive copy would come online at the second site.

Now from what what I understand I would need either a witness server or another hub transport server in place so each server know whats going on?

Basically I need to know do I need the witness server at a 3rd site or is there a better way of doing this maybe not even using DAG's?

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ok, I assume you understand my problem here I need the email system to be up all the time and if any site goes down they can point to the other and continue working.
is this not the norm? is there not a better solution? my client wants the exchange system to be virtually bomb proof as he is always travelling and sometimes there is no one in the office for weeks.

> is this not the norm
no it is not, the problem is DAG across sites was designed to be Active / Passive and not Active/Active so you cannot have live databases at the same time in 2 different sites and be able to accommodate link failures. I know it is kind of hard to believe but this is the way it is. sorry

> there not a better solution?
The solution is to have 4 exchange servers (2 in each sites) configured in 2 DAGs each one acting as active / passive

from a technical perspective you can hack the system by putting your share witness in a 3rd site as you previously said and blocking the databases to auto-mount each in a different site.

In that case we are taking manually care of the split brain syndrome but I can't say it is a fool proof solution since the slightest error would lead to split brain and huge problems and that is why i never mentioned the "solution" till now

DAC mode is configured to avoid a “split brain syndrome”. I will explain with an example. Let’s say we have a four member DAG, with two servers in each datacenter. The primary datacenter hosts the witness server and hence will always be in quorum. Now, let’s say that a power outage occurs in the primary datacenter and the exchange admin activates the secondary datacenter with an alternate file share witness.

When the power is restored in the primary site, servers come online quicker than the WAN links. Hence, when the two DAG members and witness server comes online, it has a quorum (majority) and will try to activate the databases. This will cause a “split brain syndrome” where both datacenters think that they are hosting the active databases.

DAC mode was introduced to avoid this situation. When DAC mode is enabled and the DAG members come back online, they will leverage a protocol called Datacenter Activation Coordination Protocol (DACP) before trying to mount the databases. The DACP is used to determine the current state of the DAG and whether Active Manager should try to mount the databases or not.

Now for the technical bit as to how DAC works!

Active Manager stores a bit in memory (either a 0 or 1) that tells the DAG whether it's allowed to mount local databases that are assigned as active on the server. When a DAG is running in DAC mode, each time Active Manager starts up, the bit is set to 0, which means that it isn't allowed to mount databases. When in DAC mode, the server must try to communicate with all other members of the DAG that it knows to get another DAG member to give it an answer as to whether it can mount local databases that are assigned as active to it. The answer comes in the form of the bit setting for other Active Managers in the DAG. If another server responds that it’s bit is set to 1, it means that servers are allowed to mount databases. Hence, the server starting up sets its bit to 1 and mounts its databases.

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